1. Field of the Invention
The present invention relates to an image reading device that employs a shading correction technique in an image reading operation, in particular shading correction performed in an image reading operation according to a sheet-through method, an image forming apparatus including the image reading device; and an image reading method of reading an image using the sheet-through method.
2. Discussion of the Background Art
In general, image reading devices are divided into two types: a flat-bed type and a sheet-through type. In the flat-bed type image reading device, an exposure scanning unit scans an original copy (hereinafter referred to as the document) placed on a contact glass to read the image of the document. In the sheet-through type image reading device, an exposure scanning unit of a scanner section is fixed. Thus, the sheet-through type image reading device moves a document to read the image of the document. In particular, the sheet-through type image reading device is capable of reading successive images, and therefore productivity can be improved if the number of documents read per unit of time is increased.
In the operation of reading the document image, a type of correction called shading correction is commonly performed. Shading correction corrects changes in the amount of light used to read the document image caused by variation in the main scanning direction of the illuminance provided by an exposure lamp serving as a light source used to read the document image together with a light-receiving element or by the passage of light-up time. In shading correction, a white reference plate which provides a white density reference value is read in each document reading operation, and the image output is corrected on the basis of the thus-read data.
When an image reading device capable of reading an image using either the flat-bed method or the sheet-through method performs the document reading operation using the sheet-through method, it is desirable to equalize the amount of correction carried out on image output read using the sheet-through method with the amount of correction performed on the image output read in the flat-bed method. Therefore, the image reading device moves a carriage of the exposure scanning unit to the white reference plate to perform shading correction, and thereafter moves the carriage back to a reading position to perform the document reading operation.
As described above, shading correction performed by the sheet-through type image reading device involves the process of moving the exposure scanning unit to the white reference plate and thereafter moving the exposure scanning unit back to the reading position. Consequently, the image reading device is prevented from starting the reading operation of the next document until the current shading correction is completed. As a result, in the reading of successive images, a time interval between the reading of one document and the reading of the next document is naturally longer than the time of shading correction performed in a DF (Document Feed) mode. (Hereinafter, the term “DF mode” is used interchangeably with the term “ADF (Automatic Document Feed) mode”. Further, document feeding in the DF mode will be hereinafter referred to as DF.)
Accordingly, to improve image reading productivity of the sheet-through type image reading device described above, it is desirable to reduce the time interval between the reading of one document and the reading of the next document. It is therefore desirable to reduce the time interval between successive document feeds, and to reduce the time of the above-described shading correction performed during document reading.
In light of the above-described circumstances, a known image reading device has been proposed which is capable of reading an image using either a book-read (i.e., flat-bed) method of causing an exposure scanning unit to scan a fixed document to read the image of the document or the sheet-through method of moving a document with respect to an exposure scanning unit of a scanner to read the image of the document. The known image reading device is configured to perform a successive document reading operation if a time TD is longer than a time Ts, wherein the time TD represents the time from the start of a non-stop sheet-through feeding operation by an automatic document feeder to the arrival of a leading end of a document to the reading position, and the time Ts represents the time required to perform shading correction in the DF mode in which a document is automatically fed and read.
As illustrated in the timing chart shown in FIG. 1, for example, in shading correction performed during the non-stop DF operation in accordance with the known technique described above, an instruction to perform a carriage operation is issued after the setting of a carriage driving operation and the setting of a reading operation for shading correction are performed after a time point t1 at which the reading operation of a previous document is completed. That is, the image reading device receives a reading preparation request from a controller (D1), sends a memory request to the controller (D2), and receives a memory access permission notification from the controller (D3). Thereafter, at the time point t1 at which the reading operation of the previous document is completed, the image reading device starts a pre-reading DF shading correction. Then, at a time point t2 at which the pre-reading DF shading correction is completed, the image reading device starts the reading operation of the next document. Further, at a time point t3 at which the next document is moved outside an image effective range, the image reading device completes the reading operation. According to the timing illustrated in FIG. 1, the sheet feeding operation of the next document starts after the completion of the reading operation of the previous document and before the completion of the sheet feeding operation of the previous document in the DF reading operation. Further, the DF document feeding operation is completed when the sheet feeding operation of the next document is completed.
According to the above-described timing, the time TD between the previous document and the next document in the non-stop successive sheet feeding operation corresponds to the sum of a processing time Ts of the DF shading correction and a preparation time Tp for the DF shading correction, and represents a pre-reading DF shading correction time. The time TD is represented as the following equation:TD=Tp+Ts  (1)The time TD is also represented as the following equation:TD=t2−t1  (2)
It is to be noted that productivity is determined by the processing time, that is, the shorter the processing time the greater the productivity. However, the way in which known apparatuses including that described above operate makes it difficult to reduce the processing time and improve the productivity, for reasons which are discussed in greater detail below.
FIG. 2 is a detailed timing chart illustrating the above-described time TD. As can be observed from the drawing, the preparation time Tp for the DF shading correction corresponds to the sum of a preparation time Tp1 for the reading operation for shading correction and a preparation time Tp2 for the setting of the carriage driving operation. That is, the preparation time Tp is represented as the following equation:Tp=Tp1+Tp2  (3)
Further, the processing time Ts of the DF shading correction corresponds to the sum of a time Ts1 from the start of the carriage movement to the start of the reading operation for shading correction and a time Ts2 from the start of the reading operation for shading correction to the completion of shading correction. That is, the processing time Ts is represented as the following equation:Ts=Ts1+Ts2  (4)
FIG. 3 is a flowchart illustrating a processing procedure of the processing performed in accordance with the timing illustrated in FIGS. 1 and 2. The processing procedure is divided into a shading correction setting process A including Steps S1 to S4 and a shading correction B including Steps S5 and S6. According to the processing procedure, upon completion of the reading operation of the previous document, the setting of the reading operation for shading correction (Step S1) and the setting of the timing of the reading operation for shading correction (Step S2) are performed. Then, the setting of the carriage operation for shading correction (Step S3) is performed. Thereafter, an instruction to perform the carriage operation for shading correction is issued (Step S4).
On the basis of the above-described operation instruction, the carriage operation for shading correction is started (Step S5), and the reading operation for shading correction is started (Step S6). Thereby, the reading operation for shading correction is performed. The timing of starting Step S1 corresponds to the time point t1, and the timing of completing Step S6 corresponds to the time point t2.
Thus, according to known techniques including the technique described above, in the non-stop automatic document feeding operation in the DF mode, i.e., in the successive document reading operation, the operation is performed in the order of the notification of the memory access permission (represented as D3 in FIG. 1), shading correction in the time TD, and the sheet-through reading operation. More precisely, shading correction included in the above-described operation is performed in the order of the setting of shading correction in the preparation time Tp, the carriage operation in the time Ts1, and shading correction in the time Ts2.
The processing time taken to perform shading correction in the above-described manner corresponds to the sum of the time for shading correction setting process A and the time for shading correction B. In the successive document reading operation in the DF mode, therefore, it is difficult to make the time interval between two successive documents shorter than the processing time corresponding to the sum of the times described above. Therefore, it is difficult to reduce the processing time and improve the productivity.